1. Field of the Invention
The present invention relates to a filter component that eliminates common mode noise which propagates through differential lines, and more particularly, to a technology that improves attenuation characteristics of the filter component.
2. Description of the Related Art
A known filter component includes a filter circuit for eliminating common mode noise which propagates through differential lines. As shown in the circuit diagram in FIG. 19, in a known filter component 500, passive series elements 504a and 504b are respectively connected in series with first and second lines 503a and 503b disposed between first and second differential input terminals 501a and 501b and first and second differential output terminals 502a and 502b, respectively, and a passive parallel element 505 is connected in parallel between the first and second lines 503a and 503b, thus forming a ladder differential four-terminal circuit.
More specifically, on the first line 503a between the first differential input and output terminals 501a and 502a, two inductors Lo/2 are connected in series with each other as the passive series element 504a so that they are positively coupled with each other. On the second line 503b between the second differential input and output terminals 501b and 502b, two inductors Lo/2 are connected in series with each other as the passive series element 504b so that they are positively coupled with each other. One of two capacitors Co which are connected in series with each other as the passive parallel element 505 is connected to a portion at which the two inductors Lo/2 are connected in series with each other on the first line 503a. The other one of the two capacitors Co which are connected in series with each other as the passive parallel element 505 is connected to a portion at which the two inductors Lo/2 are connected in series with each other on the second line 503b. 
A bridge capacitance Ca is connected between the two terminals of the two inductors Lo/2 connected in series with each other on the first line 503a. A bridge capacitance Ca is connected between the two terminals of the two inductors Lo/2 connected in series with each other on the second line 503b. With this configuration, in the above-described ladder differential four-terminal circuit, a bridged-T all-pass differential delay line is formed. An inductor Lc for attenuating common mode noise is connected between a ground potential and a node T between the two capacitors Co which are connected to the first and second lines 503a and 503b as the passive parallel element, thus forming a differential delay line common mode filter.
Reference signs +vd and −vd shown near the first and second differential input terminals 501a and 501b in FIG. 19 denote a power difference for impedance Zo, a reference sign Zo near the first and second differential output terminals 502a and 502b denotes a terminating impedance, and a reference sign vc denotes a common mode noise source. In the filter component 500, the above-described common mode filter may be disposed in plural in a ladder form on the first and second lines 503a and 503b which are respectively disposed between the first and second differential input terminals 501a and 501b and the first and second differential output terminals 502a and 502b. 
In the common mode filter included in the above-described filter component 500, an attenuation pole for attenuating common mode noise is formed by an LC series resonance circuit constituted by the inductor Lc for attenuating common mode noise and the capacitors Co connected as the passive parallel element 505. Differential lines (Differential signals) propagating through the first and second lines 503a and 503b are signals out of phase with each other by 180°. Accordingly, in a case in which the node T between the two capacitors Co which connect the first and second lines 503a and 503b with each other is set at a midpoint between the first and second lines 503a and 503b, the node T is a virtual ground point.
In this case, the inductor Lc connected to the node T which is grounded virtually does not function sufficiently, and thus, the attenuation characteristics for attenuating common mode noise exhibited by the LC series resonance circuit may be deteriorated. Additionally, since the LC series resonance circuit is formed by the inductor Lc connected to the virtually grounded node T, if the node T is formed at a position displaced from the ideal virtual ground point due to the restrictions of the structure of the filter component 500, the following problem may occur. A phase shift occurs in the common mode noise, and thus, common mode noise may not be sufficiently attenuated.